Interferometry refers to the superposition of electromagnetic waves in order to extract information about those waves. Low-coherence interferometry (LCI) is an interferometry technique utilizing low-coherence light sources. LCI allows for precise measurement of the amplitude and the relative phase of reflected or backscattered light.
One common type of interferometer is a Michelson optical interferometer. Michelson interferometers have a light source, a beam splitter which splits the light into a reference arm and a measurement arm and a light detector. The measurement beam is reflected from the specimen being analyzed. The time required for the measurement beam to travel back along the measurement arm and arrive at the light detector depends on the various refractive indexes in the different layers of the specimen. The reference mirror is positioned on a movable delay line to allow the time delay of the reference arm to be adjusted and matched to the time delay of the light reflecting off of the specimen and travelling through the measurement arm. The light from the reference mirror and the light from the specimen, which have multiple partial reflections at multiple delay times, are then recombined and detected. The optical distance of the reflecting regions in the specimen are determined by analyzing the recombined light beam as detected by the light detector.
Optical low-coherence reflectometry (OLCR) is an interferometry technique for one-dimensional optical ranging where the amplitude and longitudinal delay of backscattering from a specimen are resolved using a Michelson interferometer incorporating a low-coherence light source. This technique can resolve surfaces spaced by less than 10 μm and can detect optical power reflectivities as low as −136 dB. OLCR Michelson interferometers can be constructed using fiber optic components, thus minimizing their size and weight, and lowering the requirements for the alignment.
Referring to FIG. 1, an example of a basic OLCR interferometer is shown. Such an interferometer includes: a low-coherence source (A); a source-to-isolator fiber (B); an isolator (C); an isolator output fiber (D); a fiber coupler (E); a test arm fiber (F); a reference arm fiber (I); a mirror (H); a detector fiber (J); and a light detector (K). Light travels from the low-coherence source (A) through the source-to-isolator fiber (B), the isolator (C) and the isolator output fiber (D) to the fiber coupler (E), which splits the beam between the test arm fiber (F) and the reference arm fiber (I). Light in the test arm fiber (F) travels to the specimen (G), and is reflected back through the test arm fiber (F). Light in the reference arm fiber (I) travels to the mirror (H) and is reflected back through the reference arm fiber (I). The reflected light from the test arm fiber (F) and the reflected light from the reference arm fiber (I) are recombined in the fiber coupler (E). The recombined light travels through the detector fiber (J) to the detector (K). The isolator (C) prevents reflected light from interfering with light from the source (A). When the optical path length to the mirror (H) is equal to the optical path length to a reflection in the specimen (G), the reflected light from the test arm (F) and the reflected light from the reference arm (I) add coherently to produce coherence fringes (known as coherence spikes) at the light detector (K). To improve the detection of the coherence signal, the reference arm (I) can include a phase modulation device. In some embodiments, the phase modulation device has a vibrating mirror; in other embodiments, the phase modulation device has an electro-optic phase modulator. The amplitude of the coherence signal is a function of the reflection coefficient of the specimen (G). In some embodiments, the amplitude of the coherence signal can be proportional to the square root of the product of the powers in the reference and signal channels. Thus, translating the mirror (H) to vary the optical path length of the reference arm (I) allows the reflectivity profile of the specimen (G) to be mapped. When the optical path length difference is larger than the coherence length of the source (A), the coherence signal no longer exists. The coherence length of a source Lc is determined by the following equation:
      L    c    =            λ      2              n      ⁢                          ⁢      Δ      ⁢                          ⁢      λ      In this equation, n is the refractive index of the test material, λ is the average source wavelength, and Δλ is the source spectral width.
One potential application for OLCR is in the field of hydrocarbon condensation measurement systems used in the oil and gas industry. These systems must be capable of accurately quantifying small amounts of liquids under extreme temperatures and pressures. For the purposes of chemical analysis, it is also desirable that such systems be capable of accurately measuring the refractive indices of liquids.
Fiber optic systems have a number of advantages over other types of prior art liquid measurement systems, including immunity to electromagnetic interference, the ability to operate in a wide variety of environmental conditions, high sensitivity and the potential for multiplexing.
Current fiber optic liquid measurement systems are often complex and sensitive to external disturbances. Many of these systems are designed to measure only the refractive indices of test liquids and are not capable of measuring the thickness of a liquid layer. Furthermore, they require a fiber tip to be inserted into the test liquid.
Current fiber optic liquid measurement systems capable of measuring the thickness of a liquid layer require an optical reflecting surface separate from and behind the test liquid. Such systems also require a large lens surface to collect the light reflected from the test liquid.
It is therefore desirable to provide an apparatus and method for measuring the quantity and optical parameters of a liquid in a container using the principle of optical low coherence reflectometry that overcomes the shortcomings of the prior art.